The present invention relates to the fabrication and processing of plastic film, and is more specifically directed to stabilizers for maintaining a tubular extrusion of blown plastic film, that is, a bubble, on a predetermined path during the extrusion process. The invention is more particularly concerned with improvements to an external guide or stabilizer arrangement that permits the aperture of the stabilizer to be adjusted, and which minimizes or compensates for the problem of gauge, or small variations in the thickness of the film caused by the passage of the film through the stabilizer.
Polyethylene film or films of other thermoplastic materials are produced using a blown film process, in which molten thermoplastic material is fed to an annular extrusion die and the latter produces a tubular extrusion, i.e., a bubble, of the material, which is drawn upwards and solidifies into film. An air jet incorporated in the die injects air into the interior of the extrusion to inflate the bubble or tube. The film in the extrusion is initially somewhat fluid and inflates to a larger diameter with a correspondingly thinner wall, cooling and solidifying when a predetermined thickness is reached. The process is controlled and adjusted so that the film has a uniform thickness, and this basic process is well known in the art. The tubular extrusion is drawn upwards and flattened between a pair of nip rollers that are positioned above the die. Typically, a collapsing frame is provided to urge opposite sides of the tubular extrusion towards one another just before the tube reaches the nip rollers. The collapsing frame may consist of a pair of opposed arrays of horizontal collapsing boards of any of a number of well-known configurations, and one suitable arrangement is discussed in U.S. Pat. No. 4,943,226, granted Jul. 24, 1990.
As mentioned before, it is important that the thickness, as well as strength and other properties of the produced film be as uniform as possible. It is also necessary to the quality of the film that it be relatively free of weak regions and tears. For those reasons stabilizer arrangements are used to keep the tubular extrusion confined to a well-defined travel path as it proceeds from the die to the nip rollers. Guide cages or other external stabilizers can be positioned outside the bubble for this purpose. In some cases internal guide members can be disposed atop the extrusion die and within the bubble, both to prevent the tubular extrusion from drifting off the axis of travel and also to assist in the direction of air flow within the bubble.
A typical external stabilizer takes the form of a cage or frame and a series of banks of bowed arms that are positioned at intervals around the tubular extrusion. In some arrangements, each arm can carry a row of rollers made of a PTFE (Teflon) material, with the arms being positioned just out of contact with the bubble, or in light contact with it. This arrangement has limitations, due to the abrasiveness of the plastic extrusion, and due to the need to position the rollers along a curve. The use of rollers makes it difficult or impossible to supply coolant to the arms of the stabilizer, and this limits the ability to manage and control heat in the bubble. The rollers also make this type of stabilizer expensive to build and to maintain. The stabilizers can be of a variable aperture design, with an array of arms configured as a variable iris aperture, typically employing bowed arms of rollers. One example of this is shown in U.S. Pat. No. 4,815,957.
Another external stabilizer is described in U.S. Pat. No. 5,700,479, in which the stabilizer employs non-rotating bow arms of a light weight aluminum construction with wear covers of a wear-resistant, low-friction plastic material. These can be arranged either in a bank of arms or as a spiral or helical member.
A recognized problem in the blown film art is that blown film has different stretch and strength characteristics in the axial or machine direction from what is achieved in the transverse direction, i.e., circumferentially around the extrusion. This occurs because it has been difficult to control the expansion of the bubble in both directions at the same time. Ideally, the strength in the machine direction, or MD, should be the same as the strength in the transverse direction, or TD. Stabilizing the position of the extrusion or bubble helps in this regard. Also, controlling rotation of the bubble as it rises will affect these properties, with film materials of different composition or of different thickness requiring more or less rotation. Control of the temperature of the film during the extrusion process is also an important means in achieving uniformity, but little attention has been paid to finding ways for the external guide cage to assist in heat management.
One approach to an external guide cage with control of temperature in the guide arms is disclosed in U.S. Pat. No. 6,196,827. In that arrangement, there are a number of banks of extruded guide arms which can be pivoted to change the size of the iris aperture, and with diameter of the aperture changing from one level or bank of arms to the next. The use of a light-weight metal extrusion permits the arm to be of a dual tube construction so that a closed coolant flow path can be provided in each arm. A flow of coolant, such as water, can be provided through channeling in the interiors of the extruded metal arms for controlled cooling. Each arm can have a profile so as to accept a low-friction wear cover. Coolant supply and return piping and other hardware can be coupled to each arm at its pivot end.
Because of the iris shape of the aperture, there is closer contact with the bubble at some locations around the bubble than at others, and this can result in slight differences in the amount of heat transfer from the bubble to the guide arms. As a result of this, the extruded film can have slight variances in thickness in the TD corresponding to the locations where the arms are just out of contact with the film. These variations can be on the order of a fraction of a mil, but when the film is rolled onto a mandrel these gauge variations accumulate if they are all at the same location on the circumference of the bubble. Because the roll of film can comprise many thousands of turns, the gauge variation can accumulate to result-in circumferential ridges or hollows of significant size